The capacity of a two compressor or two-stage compressor system is a function of the volumetric efficiency, V.sub.e, the change in enthalpy, .DELTA. H, and the displacement efficiency, D.sub.e. In a two compressor system the flow is serially through a booster compressor (low stage) and a high stage compressor. Unloading of this arrangement is typically achieved by regulating the booster compressor. In two-stage reciprocating compressor systems the cylinders are divided between the two stages with the first stage having, typically, twice as many cylinders as the second stage. Unloading of this arrangement can be achieved by gas bypass or suction cutoff of one or more cylinders of the first stage. In fact, the entire first stage can be unloaded so that the second stage is doing all of the pumping and is being supplied at the compressor suction pressure. Since the entire first stage discharge may be bypassed to suction, this arrangement also serves to negate the capacity increase associated with the use of a economizer. In U.S. Pat. application Ser. No. 374,907, means are employed in a two-stage compression system so as to both control the temperature of the second stage discharge and to unload the compressor. Unloading the compressor is through the use of a bypass which directs the first stage discharge of the compressor back to suction. When the bypass is fully open, the second stage inlet operates at system suction pressure and second stage displacement alone must now handle the vapor generated by both the system evaporator and the economizer. This effectively reduces the vapor generated by the system evaporator to a fraction of its full load amount thus accomplishing very effective unloading.